Thermal or infrared imaging cameras are well known and have long been used in many settings to measure and/or compare the temperatures of multiple objects or the temperature profiles of a single object over a period of time. Among other uses, these instruments are frequently used in industrial applications as part of preventative maintenance programs. These types of programs typically rely on periodic inspections of the assets of a plant or facility to discover likely failures before they occur. For example, plant personnel often develop survey routes in order to routinely gather temperature data on the identified equipment.
After collecting a thermal baseline image for each piece of equipment along a route, a technician can then identify changes in thermal characteristics over the course of several inspections by comparing later thermal images of the equipment with the baseline image or other prior images. Changes in thermal characteristics can in some cases indicate the imminent failure of a part, thus allowing the technician to schedule maintenance or replacement prior to failure.
In a simple case, a technician can visually compare two or more thermal or infrared images to determine changes in thermal characteristics occurring over a period of time. Visual comparison of two infrared images can at times be difficult because most infrared images are by their very nature less sharp compared to visible images which are a result of reflected visible-light. For example, considering an electric control panel of an industrial machine which has many electrical components and interconnections, the visible image will be sharp and clear due to the different colors and well defined shapes. The infrared image will appear less sharp due to the transfer of heat from the hot part or parts to adjacent parts. Also, coloring, lettering and numbering clearly shown in the visible image will not appear in the infrared image.
Infrared images often have a low resolution when compared with visual images and may be especially difficult to interpret and compare when part of the scene and image exhibit a low infrared image contrast. Accordingly, it may be difficult to correlate features and objects within two infrared images because object and feature boundaries can be difficult to discern in low resolution. In addition, the thermal image profile may have changed between image captures such that features present in one infrared image are now invisible (i.e., the same temperature as the scene background) in a second infrared image of the same scene. Thus, while a change in temperature may be suspected, it can be difficult to determine which objects or parts of objects within the scene exhibit the thermal change, in part because it is difficult to discern which objects in the first image correspond with objects in the second infrared image.
To aid a visual comparison of infrared images, some infrared imaging cameras allow the operator to capture a visible-light image (often called a “control image”) of the scene using a separate visible-light camera built into the thermal imaging camera. Some of these thermal imaging cameras allow a user to view the visible-light image side-by-side with the infrared image. Some thermal imaging cameras may also provide simultaneous viewing of the infrared image and the visible-light image overlapping each other and blended together. For example, Fluke Corporation's FlexCam® series of cameras incorporates a software feature called IR-Fusion®, which allows a user to blend the infrared and visible-light images together at any ratio from 100% visible to 100% infrared. Such features provide the operator with a visible-light image reference for each infrared image, thus making visual comparison of the infrared images somewhat easier.
Image processing software may also be used to compare two or more infrared images, and can, in many instances provide a much more informative and quantitative comparison of two infrared images than a mere visual inspection. For example, image processing software may subtract a later infrared image from an earlier infrared image of the same scene to determine whether changes in the thermal characteristics of the scene have occurred.
A further complexity for image comparison can result from unaligned or misaligned infrared images. In some cases, a fixed or stationary infrared imaging camera may be used to capture highly registered successive thermal images of a scene. Often, though, multiple infrared images are captured using a handheld thermal imaging camera. Accordingly, images of the same scene captured at different times may show different views of the same scene. For example, successive infrared images may have slightly different perspectives of the scene, may be captured with different zoom factors, and/or may be translated (e.g., horizontally and/or vertically) with respect to each other. These types of misalignments between infrared images can make thermal images difficult to compare to one another, both visually and through software comparisons in which computed comparison data is not very useful.